Micro-acoustic waveguide

ABSTRACT

Apparatus is disclosed including a waveguide structure for controlling propagation of elastic waves including a rigid supporting substrate for suppressing propagation and an overlayer of elastic material for sustaining propagation and having a closed, defined, reflecting boundary surface for confining the elastic waves in the transverse dimensions.

United States Patent [191 Waldron et al.

[ Jan. 29, 1974 MICRO-ACOUSTIC WAVEGUIDE [75] Inventors: Richard A. Waldron, Ipswich,

England; Ernest Stern, Concord, Mass.

[73] Assignee: Massachusetts Institute of Technology, Cambridge, Mass.

[22] Filed: May 24, 1972 [21] Appl. No.: 256,469

[52] US. Cl 333/30 R, 333/10, 333/72, 333/95 R [51] Int. Cl H03h 7/30, I-l03h 9/30 [58] Field of Search 333/30 R, 72, 10, 95 R; 330/55 [56] References Cited UNITED STATES PATENTS 3,406,358 10/1968 Seidel et al. 333/10 X 3,409,848 11/1968 Meitzler et al 333/30 R X 3,464,033 8/1969 Toumois 3,488,602 1/1970 Seidel et al. 333/30 R X Primary Examiner-Rudolph V. Rolinec Assistant Examiner-Marvin Nussbaum Attorney, Agent, or Firm-Arthur A. Smith, Jr.; .10- seph S. Iandiorio ABSTRACT Apparatus is disclosed including a waveguide structure for controlling propagation of elastic waves including a rigid supporting substrate for suppressing propagation and an overlayer of elastic material for sustaining propagation and having a closed, defined, reflecting boundary surface for confining the elastic waves in the transverse dimensions.

17 Claims, 12 Drawing Figures PATENTEUJAHZSIQN 3,789,327

;\nt.H1UF5 2 TOP SURFACE OF OVERLAYER 1 Hummw.

LSURFACE OF SUBSTRATE 0.4 0.6 0.8 1.0 24 AMPLITUDE OF VIBRATION PATENTED m. 2 9 m4 snz'uanrs POSITIONS OF EDGES OF OVERLAYER* PATENTEDJMIZQ 1924 SHEET 3 0F 5 USEFUL SINGLE-MODE OPERATIONAL RANGE OVERLAYER 72 SUBSTRATE COUPLER 1 MICRO-ACOUSTIC WAVEGUIDE FIELD OF INVENTION This invention relates to a waveguide for elastic waves, and more particularly to a waveguide which confines the waves in the transverse direction in a closed defined boundary.

BACKGROUND OF INVENTION Elastic waves and particularly surface acoustic waves propagate in solids at speeds which are typically times slower than electromagnetic waves. The slower speed of these waves makes them suitable for use in delaying functions such as could be implemented by delay lines; a delay of several microseconds may be achieved in a centimeter of acoustic delay line whereas a similar electromagnetic delay line would require a kilometer. This technology has continued to expand and is known by many as microsound, a name inspired by the field of microwave technology which is analagous in many ways. More background in microsound technology may be gained from the Special Issue on Microwave Acoustics, IEEE transactions on Microwave Theory and Techniques, November 1969, Volume MTT-l7, Number l l.

Waveguides were introduced into this new field to reduce spreading of the beam, improve the accuracy of a beam path between a source and receiver, and control the direction ofa beam to permit close packing and wider choice of beam paths.

One type of waveguide includes a substrate of, for example, quartz crystal and a thin overlayer of a metallic substance such as gold. See Elastic Surface Waves Guided by Thin Films: Gold on Fuzed Quartz, L. R. Ad kins and A. J. Hughes, P. 904, Vol. MTT-l7, November 1969, IEEE Transactions on Microwave Theory and Techniques. The substrate is relatively large but the overlayer is approximately one wavelength wide and one-thirtieth to one-sixtieth of a wavelength thick. The elastic waves move in the substrate; the metallic overlayer functions to concentrate somewhat those elastic waves in the area of the substrate beneath the overlayer. Actually, the wave energy is quite widespread in the substrate and may be still substantial as far as an overlayer width from each edge of the overlayer. This type of guide tends to have substantial losses; for example, a bend of overlayer widths radius would have high loss but with a radius of fifty overlayer widths the losses may be reasonable, it has been said. Since an important feature of microsound is its potential for realizing components and systems of substantially smaller size, a waveguide that requires a bend with a radius no less than fifty times its own width, i.e.

one wavelength, severely detracts from its attractiveness.

Another type of waveguide that has been suggested, Microsound Surface Waveguides, Ash, De La Rue and Humphryes, P. 882, Vol. MTT-l7, IEEE Transactions on Microwave Theory and Techniques, November 1969, employs a substrate on which is grown an epitaxial layer of the same material. At higher frequencies the energy is more or less confined in the epitaxial layer but there may be more than one mode and at lower frequencies a large portion of the energy is present in the substrate. Various other types of waveguides including one or more grooves in the overlayer, lenses and other means have met with indifferent sucess.

Experience suggests that many analogues of microwave devices could be fabricated for microsound systems if a waveguide could be made which provides a closed, sharply defined boundary surface for confining elastic wave energy in the transverse dimensions. Then such analagous microsound components as T junctions, mitred corners, filters, tapers, directional couplers, hybrid couplers, circulators, isolators, Faraday rotators, and others become a reality.

The problem of a proper waveguide is made more complex by the fact that for practical applications ofsignal processing large bandwidths would be necessary, which in turn would require frequencies of many hundreds of megacycles or even one or two thousand megacycles. The wavelength at these frequencies is approximately ten microns to one or two microns. Since, typically the components would be so small that they could'not be self-supporting as with conventional microwave devices.

SUMMARY OF THE INVENTION It is therefore an object of this invention to provide a new and improved waveguide for elastic waves which confines the elastic wave energy within highly reflecting surfaces in the transverse dimensions.

lt is a further object of this invention to provide such a Waveguide in which the elastic wave energy is confined within a closed, defined boundary surface.

It is a further object of this invention to provide such a waveguide capable of operating in a single mode.

It is a further object of this invention to provide such a waveguide which is possessed of mechanical strength even though fabricated to service waves having wavelengths in the one to ten micron or even submicron range.

The invention features a waveguide structure for controlling propagation of elastic waves. There is a rigid supportingsubstrate for suppressing propagation ofthe waves. The substrate supports an overlayer of elastic material in which propagation of the waves is sustained and which has a closed, defined, reflecting boundary surface for confining the eleastic waves in the transverse dimensions.

DISCLOSURE OF PREFERRED EMBODIMENTS Other objects, features and advantages will occur from the following description of preferred embodiments and the accompanying drawings, in which:

FIG. 1 is an axonometric, cross-sectional, diagrammatic view of a waveguide including an elastic overlayer and rigid substrate according to this invention.

FIG. 2 shows a graph of the variation of the amplitude of an elastic wave with respect to the thickness dimension of the overlayer and substrate superimposed on a cross-section of an overlayer and substrate.

FIG. 3 shows a graph of the variation in the X and Z amplitude components of an elastic wave with respect to the width dimension of the overlayer and substrate superimposed on a cross-section of an overlayer and substrate.

FIG. 4 is an illustrative sketch of a graph showing variations of the ratio of energy in substrate to energy in overlayer with respect to the ratio of overlayer thickness to width.

FIG. 5 is a graph showing the variation of phase characteristics of S, D and L mode elastic waves for various ratios of density and shear modulus for the overlayer and-substrate with a particular ratio of overlayer thickness to width.

FIG. 6 is an enlarged graph similar to that of FIG. for a different ratio of overlayer thickness to width showing increased spacing between modes.

FIG. 7 is a diagrammatic cross-sectional view of a directional coupler according to this invention.

FIG. 8 is an axonometric diagram of a waveguide with a mitred corner according to this invention.

FIG. 9 is an axonometric diagram of a T-junction in a waveguide according to this invention.

FIG. 10 is an axonometric diagram of a T-junction with matching elements in a waveguide according to this invention.

FIG. 11 is an axonometric diagram of a filter with matching elements in a waveguide according to this invention.

FIG. 12 is an axonometric diagram of a hybrid coupler in a waveguide according to this invention.

The invention may be accomplished in a waveguide having an elastic overlayer supported on a rigid substrate. The overlayer may have any desired crosssection shape which can be properly supported on the substrate. Ideally, for maximum effectiveness of the waveguide the substrate is prefectly rigid and the overlayer is not, so that the elastic waves are easily propagated in the elastic overlayer but completely suppressed in the rigid substrate. Practically, for effective operation of the waveguide, the substrate has a shear modulus u which is much greater than the shear modulus u of the overlayer. Typically the ratio of u lu may be 0.1. In addition to the relative rigidity of overlayer and substrate, a second consideration for effective waveguide operation is the relative values of the transverse dimensions of thickness 17 and width 0 of the overlayer. Generally, width :1 is in the order ofa wavelength and thickness b is less than 0. Typically the ratio of 12/0 may be 0.4.

That the device may be made to operate as an effective waveguide and that the overlayer may be made to contain all or nearly all of the elastic wave energy, while little or no energy propagates in the substrate, by optimizing the values of b and a and u and 14 is explained in Microsound Waveguides and Waveguide Components, IEEE Transactions on Sonics and Ultrasonics, October 1971 by .the inventor Richard A. Waldron. Particularly equation (Z0):

of that article sets forth an expression for the ratio of the energy in the substrate W to the energy in the overlayer W,. Since a is implicit in s, and s it follows from this expression that the efficiency of the waveguide is a function ofthe ratio 11/0 and of the ratio Il /U For one solution of equation in which 12/11 is 0.4 and 14 /14 is 0.1 there is shown to be 3% percent of the energy in the substrate and 96% percent in the overlayer. Thus the wave energy is confined in the transverse dimensions of the overlayer within the closed defined boundary surface and a waveguide comparable to a microwave waveguide is achieved.

The invention may be embodied in a waveguide structure 10, FIG. 1, including an overlayer 12 of material having a shear modulus u, supported on a substrate 14 of material having a shear modulus of 11 In FIG. 1

overlayer 12 is shown having a rectangular crosssection of thickness b and width a. Typically width a is a wavelength and width b is less than a, whereas similar dimensions of substrate 14 may be orders of magnitude larger. If overlayer 12 is cylindrical than b and a are equal to the diameter. Similar dimensional accommodations may be necessary for other shapes.

Typical combinations of materials which have the proper ratio of u /u for use in the waveguide include cadmium sulfide as the overlayer and spine] as the substrate in the neighborhood of one hundred megacycles per second; lead glass as the overlayer and alumina as the substrate in the neighborhood of ten megacycles per second; and zinc oxide as the overlayer and diamond as the substrate in the neighborhood of a thousand megacycles per second. An illustrative waveguide operating at low frequencies, i.e. twenty cycles per second, is made with an overlayer of gelatin and substrate of aluminum.

In FIG. 1 elastic waves propagating along waveguide 10 are confined withinoverlayer 12 by the closed surface defined by the boundaries of side surfaces 16, 18 and top surface 20 with the surrounding medium, such as air, and by the boundary of bottom surface 22 with mating surface 24 of substrate 14. In the particular example cited with reference to equation (20) aforementioned i.e. 12/11 0.4, u /u 0.1, wherein 3% percent of the energy was in substrate 14, 0.48 percent of the energy in the substrate or 0.016 percent of the total energy in the system is in the spaces 26 and 28 on either side of the space 30 and the rest of the 3% percent is in space 30 beneath overlayer 12.

The variation in wave energy in overlayer 12 and substrate 14 along the thickness dimension b, i.e. in the y direction, is illustrated in FIG. 2 which is a combination of a graph superimposed on a cross-section of the waveguide 10 of FIG. 1. At the top surface 20 of overlayer 12 where y b the amplitude of vibration, of which wave energy is a function, is maximum as shown by curves 32, 34 and 36. All three curves show amplitude gradually decreasing as the mating surfaces 22 and 24 of overlayer l2 and substrate 14 are approached. Beneath surfaces 22, 24 in substrate 14 curves 34, 36 indicate that the amplitude falls off more rapidly with increasing penetration into the substrate. Curve 32 represents the case where substrate 14 is a perfectly rigid material, u /u 0, in which elastic wave propagation is completely suppressed. Curves 34 and 36 represent the case where 14 /11 0.1, 0.3, respectively and the substrate is not perfectly rigid but approximately ten times and three times more rigid, respectively, than the overlayer.

The variation in wave energy in overlayer 12 and substrate 14 along the width dimension a, i.e. in the x direction, and along the propagation direction of the waveguide is illustratedin FIG. 3. As in FIG. 2 the graph in FIG. 3 has been drawn superimposed on a cross-section of waveguide 10. Curve 40 depicts that the amplitude of displacement in the width dimension in overlayer 12 is maximum at the center of overlayer 12 and minimum at the outer surfaces 16 and 18 and curve 42 depicts that the amplitude of displacement in the direction of propagation varies from a maximum at one surface 16 through a minimum at the center and to a maximum in the other direction at the other surface 18. Curve 44 depicts the same general behavior as curve 40 but occurring in substrate 14 and illustrates thatenergy in substrate 14 is similarly distributed but ""'as curve 42 but occurring in substrate 14 and illustrates that energy in substrate 14 is similarly distributed but does spread beyond space 30 into adjacent spaces 26 and 28 as represented by portions 52, 54 of curve 50. Curves 40, 42, 44 and 50 were derived in the case where u /u 0.1 and b/a 0.4.

Application of equation (20), supra, for various values of b/a indicates that, curve 60, FIG. 4, for b/a in the range from to nearly 0.2 the percentage of wave energy in the substrate W /W, is 10 percent and above. At a value of 0.2 for b/a the percentage decreases rapidly, i.e. at 0.3, 6 percent; ot 0.4, 3% percent; at 0.5, 2 percent and at 0.6, l /2 1 percent. Actually at values of b/a in the neighborhood 0.6 and greater the behavior is unclear but predicted to follow the pattern indicated by the dashed line.

Another feature of the waveguide of this invention is the ability to control mode. selectivity or mode bandwidth as a function of the b/a value. Previously whenever necessary the discussion has been limited to the operation of the waveguide in one mode S as for example in respect to FIG. 2. But in microsound as in microwave applications waves may propagate in more than one mode, see Mode Spectrum ofa Microsound Waveguide Consisting of an Isotropic Rectangular Overlay on a Perfectly Rigid Substrate, by the inventor Richard A. Waldron, P. 8, Vol.- SU-I8, January 1 IEEE Transactions on SonicsaiidUltrasonics. In FIG. curves S S and S of the S mode, curves L and L of the L mode and Curve D of the D mode illustrate the variation in normalized phase constant B of each of those modes as a function of the value b/k where is the wavelength of bulk shear waves in the material of the overlayer. The curves in FIG. 5 are for the case of a perfectly rigid substrate u,/u 0 and b/a 0.2. Note particularly that the spread at zero [3 between S and L is approximately 0.31 0.27 0.04. Compare this spread of 0.04 with that in FIG. 6 where b/a is 0.4. There the spread between S and L is approximately 0.43 0.32 O. I l. Thus the useful single mode operational range has been significantly increased by increasing b/a from 0.2 to 0.4, an increase which is wholly compatible with increasing the percentage of wave energy confined in the overlayer.

With the waveguide structure of this invention many different components may now be constructed such as T-junctions, mitred corners, filters, tapers, directional couplers, hybrid couplers, circulators, isolators, Faraday rotators and more.

A directional coupler 70, FIG. 7, may be fabricated by interconnecting two overlayers l2 and 12 on substrate 14 with a coupler overlayer 72 to permit wave energy to pass between overlayers 12 and I2. Coupler overlayer 72 is necessary because the leakage of energy into the substrate from overlayers l2 and 12 is so low that it cannot be relied on to accomplish the coupling as is possible in certain prior art microsound waveguides. The thickness of coupler overlayer 72 is determined by the amount of energy desired to be coupled.

A mitred corner guide, FIG. 8, may be constructed now with the waveguide structure of this invention because losses due to discontinuity such as at edges 74, 76 are much less of a problem when the waves are confined in the overlayer 12. The value of b/a must be optimized for individual wavelengths.

Similarly a T junction 78, FIG. 9, between intersecting overlayers 12, and a turnable T junction 80, FIG. 10 with matching slot 82 andstubs 84, 86 may be made without suffering great losses. In FIGS. 8, 9 and 10 the angle of intersection is illustrated as 90 but that is not a necessary limitation of the invention as other angles may also be used.

A microsound filter 90, FIG. 11, may include on an overlayer 12 resonant segments 92, 94, 96 similar to resonant cavities in microwave components for increasing the frequency selectivity of the filter. The number of segments used may be more or less than three and may be of a different form than those illustrated. Further, different segments in a filter may be of different forms.

A hybrid coupler 100, FIG. 12, is constructed with two overlayers 12, 12b converging at junction 102 then diverging 12a, 12b at mitred corners 104, I06 separated by slot 108. Increasing and decreasing the length of junction 102 controls the crossover of energy from 12a to 12b and 12b to 12a. Decreasing the length of junction 102 reduces the energy crossover through it.

Other advantages will occur to those skilled in the art and are within the following claims:

What is claimed is:

l. A waveguide structure for controlling the propagation of elasticwaves comprising a rigid supporting substrate for suppressing propagation of elastic waves and an overlayer of elastic material for sustaining propagation of elastic waves and having a closed, defined, refleeting boundary surface for confining elastic waves in its transverse dimensions.

2. The structure of claim 1 further including a second overlayer on said substrate proximate the first and a coupler overlayer on said substrate interconnecting the two overlayers for controlling coupling of elastic wave energy between them.

3. The structure of claim 2 in which said coupler overlayer has a thickness less than that of the two overlayers.

4. The structure of claim 1 further including a second overlayer on said substrate transverse to the first and a mitred corner overlayer on said substrate interconnecting the two overlayers.

5. The structure of claim 1 further including a second overlayer on said substrate intersecting said first overlayer to form a T junction.

6. The structure of claim 5 in which said T junction includes a tuning slot.

7. The structure of claim 5 in which said T junction includes at least one tuning stub.

'8. The structure of claim 1 in which said overlayer includes a resonant segment extending transversely of said overlayer in the width dimension and forming a filter for increasing the frequency selectivity of the waveguide.

9. The structure of claim 1 further including a second overlayer, spaced from the first said overlayer on'the substrate, which converges toward, merges with, and diverges from the first said overlayer to form a junction at the merger for conducting to one overlayer elastic wave energy introduced by the other overlayer.

10. The waveguide structure of claim I in which the width of said overlayer is greater than its thickness.

15. The waveguide structure of claim 1 in which the ratio of the shear modulus u, of said overlayer to the shear modulus L42 of said substrate is less than 0.5.

16. The waveguide structure of claim 1 in which said substrate is more than twice as rigid as said overlayer.

17. The waveguide structure of claim 1 in which the shear modulus a of the substrate is more than twice that of the shear modulus u, of the overlayer.

UNITED STATES PATENT OFFICE ,t v CERTIFICATE OF CORRECTION Patent No. 3,789,327 Dated January 29, 197

Richard A. Waldron and Ernest Stern It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Insert as a paragraph under the section "Background of Invention", column 1:

--The invention herein described was made in the course of work performed under a contract with the United States Air Force.

Signed and sealed this 16th day of July 197A.

(SEAL) Attest:

MCCOY M. GIBSON, JR. 0. MARSHALL DANN Attesting Officer Commissioner of Patents FORM Po-1oso (IO-69) USCOMWDC 0376mm,

i ".5, GOVERNMENT PRINTING OFFICE IQIS 0-356-33l, 

1. A waveguide structure for controlling the propagation of elastic waves comprising a rigid supporting substrate for suppressing propagation of elastic waves and an overlayer of elastic material for sustaining propagation of elastic waves and having a closed, defined, reflecting boundary surface for confining elastic waves in its transverse dimensions.
 2. The structure of claim 1 further including a second overlayer on said substrate proximate the first and a coupler overlayer on said substrate interconnecting the two overlayers for controlling coupling of elastic wave energy between them.
 3. The structure of claim 2 in which said coupler overlayer has a thickness less than that of the two overlayers.
 4. The structure of claim 1 further including a second overlayer on said substrate transverse to the first and a mitred corner overlayer on said substrate interconnecting the two overlayers.
 5. The structure of claim 1 further including a second overlayer on said substrate intersecting said first overlayer to form a T junction.
 6. The structure of claim 5 in which said T junction includes a tuning slot.
 7. The structure of claim 5 in which said T junction includes at least one tuning stub.
 8. The structure of claim 1 in which said overlayer includes a resonant segment extending transversely of said overlayer in the width dimension and forming a filter for increasing the frequency selectivity of the waveguide.
 9. The structure of claim 1 further including a second overlayer, spaced from the first said overlayer on the substrate, which converges toward, merges with, and diverges from the first said overlayer to form a junction at the merger for conducting to one overlayer elastic wave energy introduced by the other overlayer.
 10. The waveguide structure of claim 1 in which the width of said overlayer is greater than its thickness.
 11. The waveguide structure of claim 1 in which the width of said overlayer is equal to its thickness.
 12. The waveguide structure of claim 1 in which the ratio of the thickness b to the width a of said overlayer is in the range 0.2 to 0.6.
 13. The waveguide structure of claim 1 in which the width of said overlayer is approximately a wavelength.
 14. The waveguide structure of claim 1 in which the width dimension is less than ten times the thickness dimension of said overlayer.
 15. The waveguide structure of claim 1 in which the ratio of the shear modulus u1 of said overlayer to the shear modulus u2 of said substrate is less than 0.5.
 16. The waveguide structure of claim 1 in which said substrate is more than twice as rigid as said overlayer.
 17. The waveguide structure of claim 1 in which the shear modulus u2 of the substrate is more than twice that of the shear modulus u1 of the overlayer. 